Research On The Current Overshoot In Open Process Of High Voltage Series-connected IGBT Valve Modulars And Its Suppression

With the advantages of simple structure, easy to control and small occupied area, the converters based on series-connected IGBTs are widely used in HVDC flexible. And the Cascade Two Level converters (CTL) which consist of series-connected IGBTs valve are seen as the main equipment in the HVDC grid with the advantages of high voltage and capacity. The IGBT-series tehnology is significant for the development of HVDC grid and solving the energy problems.The converters based on series-connected IGBTs are different from the common converters:one hand, they have to solve the voltage-balance problem; on the other hand, the current over-shoot is also different. Firstly, its mechanism is more complex. For the reasons of high voltage and high frequency, the stray parameters will also affect the current over-shoot besides of the snubber circuits and the Fast Recovery Diode (FRD). secondly, its suppression is more flexible. The series-connected IGBTs are driven by Active Voltage Control system(AVC), which make it possible to suppress the effect from the snubber circuit and the FRD, but it is studied by none papers. For the reasons above, the work in this paper is as following:Analysis the impacts of over current in series connected IGBTs valve, which will lead to IGBTs’self-locking effect and working out of SOA and reduce the life of IGBTs. And it has bad effect on the safe and stable operation of IGBTs.Study the reasons of current over-shoot of IGBTs, which include the stray parameters in converters, the reverse recovery of diodes and the snubber circuits. The mechanism of over current caused by the three factors is also studied in detail and some simulations are carried out to analysis the impacts deeply at last.The High Frequency (HF) damper is introduced to suppress the impacts of stray parameters in this paper. Then the HF damper is analyzed theoretically and designed in detail. At last, the results of simulations and experiments show that the HF damper can suppress the overshoot current resulted by stray parameters effectively without affecting the normal operation of converters.The Active Voltage Control is introduced to reduce the effects of snubber circuits and its reference waveform is optimized to enhance its voltage balancing and further decrease the effects of snubber circuits on the current over-shoot. At last, some simulations are carried out and the results show the effectiveness of the optimal AVC.The reference waveform are further optimized to restrain the effect of FRD and the results of simulationS show its effectiveness.